1. Field of the Invention
The invention relates to a transmission system comprising a transmitter and a receiver for transmitting and receiving an information signal, a video signal in particular, in a digital format obtained by means of differential pulse code modulation (DPCM). The transmitter of this system includes a DPCM coding arrangement and the receiver includes a DPCM decoding arrangement.
2. Description of the Prior Art
In one type of DPCM coding arrangement, a prediction signal is continuously subtracted from the information signal to generate a difference signal which is first quantized and thereafter encoded. This prediction signal represents an estimated version of the information signal. It is derived from the quantized difference signal which, for that purpose, is applied to an integrating network. The encoding action results in that quantized difference signal, being converted into a DPCM signal consisting of a sequence of code words which occur at a predetermined rate f.sub.s, alternatively denoted the sampling rate. The inverse quantity 1/f.sub.s is sometimes denoted the sampling period and is represented by the symbol T.
The code words generated by the DPCM coding arrangement are transmitted to the DPCM decoding arrangement via a transmission medium. In the decoding arrangement the received codewords are decoded, as a result of which the quantized difference signal is obtained again. This difference signal is also now applied to an integrating network which produces an output signal in response thereto, the shape of which accurately corresponds with the shape of the original information signal. To accomplish this, this integrating network of the DPCM decoding arrangement is of the same construction as the integrating network of the DPCM coding arrangement and both integrating networks have the same time constant.
The integrating network of the DPCM decoding arrangement has the property that each received codeword contributes during a predetermined time interval to the formation of the output signal. As a consequence, should a codeword be disturbed on the transmission path, this disturbance negatively affects this output signal during that time interval. This time interval is usually many times longer than the sampling period T and is closely related to the time constant of the integrating network. If this time constant is infinitely large (in this connection the term "ideal integrator" is sometimes used), the output signal of this integrating network will never obtain the proper value again after the occurrence of a transmission error. In such a case, as applied to a TV video signal, it is customary to set the integrating networks of the coding and decoding arrangements to a fixed value at the end of each TV line.
When the time constant becomes smaller, (in this connection the term "leaky integrator" is sometimes used) the time interval also becomes smaller. A decrease of the time constant is accompanied, however, by a deterioration of the quality of the TV picture. The highest picture quality is obtained when ideal integrators are used. If leaky integrators are opted for, the leakage factor will always be a compromise between the length of the time interval (that is to say the speed at which a transmission error is eliminated) and the loss in picture quality.
In order that, in a DPCM transmission system in which ideal integrators are used, the influence of a disturbed code word is eliminated in a very short period of time, References 1, 2 and 3 propose to add in the transmitter an error reduction signal to the DPCM signal. This error reduction signal is then generated by an error reduction circuit which comprises a quantizing arrangement and a coding arrangement connected in cascade therewith. The information signal to be transmitted, or an estimated version thereof, is applied to this error reduction circuit.
In the receiver of this DPCM transmission system, an error reduction signal is subtracted from the received sum signal to generate the original DPCM signal. Also the error reduction signal generated in the receiver is generated by an error reduction circuit which is likewise formed by a quantizing arrangement coupled in cascade therewith. This error reduction circuit is supplied with a signal which is generated in the DPCM decoding arrangement and which, in the case of an undisturbed transmission, corresponds, to the best possible extent, to the error reduction signal generated in the transmitter.